The machine tool control and related arts are quite well developed and have given rise to a myriad of patents disclosing systems for controlling the motion of a movable member such as a cutting tool or work piece. Exemplary prior art systems are disclosed in the following U.S. Pat. Nos.:
4,096,563 4,396,987 4,587,607
4,215,406 4,433,383 4,598,380
4,251,858 4,472,783 4,616,326
4,262,336 4,473,883 4,639,653
4,288,849 4,531,182 4,639,878
4,306,292 4,550,366 4,661,912
4,332,012 4,550,375 4,663,730
4,347,564 4,564,913 4,672,550
4,396,973.
Typically, such prior art systems cause a movable member to traverse a path described by an input NC part program which includes data, either incremental or absolute, defining path segments, as well as a nominal progression rate along the path. The input part program may be presented in the form of punched paper tape or other physical media or may be represented electronically. The input data is typically supplied to a command position synthesizer which generates continuously updated command position signals, each defining the desired instantaneous position for one of the axis actuators. More specifically, the command position signals are supplied to multiple position servo loops, each such loop controlling a different axis actuator. Typically, each position servo loop controls an axis actuator by producing a motor speed command proportional to the difference (error) between the commanded (or desired) position and an actual (or sensed) position. A speed regulating loop is generally associated with each position servo loop to improve the faithfulness with which the actuator reacts to the motor speed command. Without the speed regulating loop, the precision of the actuator would not only depend on position error, but on velocity error as well.
When operating at high speeds, such prior art systems generally exhibit inaccuracies on contoured paths because of servo lag error typically associated with position loops which create axis motion only in response to an error, i.e. difference between the commanded position and actual position. Feed rate along the path typically must be restricted to maintain the error within tolerable levels, thus compromising machine productivity and causing longer than optimal point to point positioning times.
More specifically, typical prior art systems use a "boxcar" interpolation scheme in which step-function position-change commands are input to a Type 1 position servo loop which acts as a filter. In this type of system, axis motion results from the error, or "lag" between the commanded and actual position. This error does not produce machining error on straight-line paths, but does result in corner and circle undercutting. The amount of undercut is related to the speed of traversal along the path; for example, a 1" diameter circle would be 0.020" undersize if executed at 100 IPM on a machine with a response of 16.7 rad/sec. Moreover, in addition to undercutting, conventional approaches exhibit relatively slow response times, and fail to exploit the full acceleration capabilities of the machine.
A further machine performance limitation imposed by conventional control systems is the limited part-program data input and processing rates which they are able to sustain. A complex path will consist of numerous short line and arc segments, requiring a significant amount of input data to define the path. Performance is then bounded by the read speed of the input system and the linear and circular data block processing time.